Mormon church president Thomas S Monson said that the faith will lower its minimum age requirement for missionaries from 19 to 18 for men and from 21 to 19 for women.
Monson made the announcement at the faith's 182nd Semi-annual General Conference in Salt Lake City.
He said the historic change is expected to significantly increase the missionary force of The Church of Jesus Christ of Latter-day Saints - more than 55,000 worldwide.
The previous age requirements had been in effect for decades. Men serve full-time, two-year missions, while women serve 18-month missions.
Mr Monson stresses young people will not be required to serve at the earlier ages, but it is an option available to them.
Mr Monson also announced that the church will build temples in Tucson, Arizona, and Arequipa, Peru.